Ceramic materials exhibit various technologically important optical, mechanical and electronic properties. As dielectric materials, ceramics have a wide range of applications including uses in high energy particle beam accelerators, fusion experiments, free electron lasers and high powered lasers, high powered X-ray and microwave tubes, electrostatic generators, pulse power switches, space platforms, satellites, and solar arrays.
Organometallic sol-gel derived optics materials with excellent transmission properties and low thermal expansion coefficients have been synthesized. Hench, et al. Proc. S.P.I.E.--Int. Soc. Opt. Eng. (1988) 76; Hench, Mater. Res. Soc. Symp. Proc. (1988) 125:189, Hench, N.A.T.O. ASI Ser., Ser. E. (1985) 92:259. Ultra-low thermal expansion glass has been produced from transition metal-containing SiO.sub.2 glasses, Shoup, U.S. Pat. No. 4,786,618.
Metal oxides have been used in the sol-gel process for encapsulation of metals in ceramic matrices. Roy et al., Mat. Res. Bull., (1984) 19:169, Roy et al., Mat. Res. Soc. Symp. Proc., (1984) 32:347; Subbanna et al., Mat. Res. Bull., (1986) 21:1465. The standard method of producing metal-containing ceramic materials involves 1) dissolution of a metal salt and Si(OR).sub.4 in an aqueous/alcoholic solvent at a pH of less than 3 or greater than 9 to form a polymer gel; 2) drying of the gel to a xerogel, 3) calcination by heating to approximately 500.degree. C. in air, and finally 4) reduction of the metal salt in hydrogen at 300.degree. C. to 900.degree. C. to produce the metal having a metallic(0) oxidation state.
Trialkylsilanes (R.sub.3 SiH) have been used to reduce transition metal salts to metals (0) in solution. It is also known in the art that triethoxysilane decomposes in aqueous solutions to form polysiloxane. In addition, the deposition of palladium onto a siloxane polymer has been achieved by using palladium(II) acetoacetonate and tetraethoxysilane followed by calcination. Schubert et al., Chem. Mat., (1989) 1:576.
Thus the conventional approach for incorporation of a metal into a ceramic matrix requires harsh (high temperature) reaction conditions. Moreover, reduction of the metal salt occurs only after the calcination process, meaning metal reduction takes place on solid, calcinated material. Under these conditions uniform reduction of the metal salt cannot be ensured. Nonhomogeneous calcinated products are obtained due to the fact that entire metal particles remain as cations, or metal particles are reduced only on their outer surfaces to the metallic(0) state. This stems from the fact that, in general, reactions on solids are far less efficient than reactions in solution. Moreover, calcinated ceramic materials are difficult to work with in that they cannot be easily shaped, molded or used to cast thin films.
To overcome these deficiencies of the prior art, applicants sought and found a method of preparing a ceramic matrix material containing a homogeneous dispersion of metal particles using sol-gel methods without calcination, in which the metal is completely reduced to the zero oxidation state as it is dispersed in the ceramic matrix material. In achieving this goal, applicants have also discovered that the resulting sol-gel ceramic matrix material is not only useful for known applications of ceramic compositions, but functions as a highly reactive and selective catalyst for hydrogenation and oxidation reactions. This is unusual since, in catalyst development, one normally achieves either a highly reactive or a highly selective catalyst.
In general, heterogeneous catalysts have been found to be more reactive than their homogeneous counterparts. Heterogeneous catalysts are often more resilient to air and moisture and they may exhibit longer catalytic lifetimes that homogeneous catalysts. Moreover, heterogeneous catalysts can be removed from a reaction system by simple filtration, and can therefore be used in flow systems, which makes them particularly attractive for industrial processes. However, heterogenous catalysts are often inferior in terms of selectivity. Accordingly, the development of homogeneous organometallic complexes for selective hydrogenation has emerged rapidly over the past two decades. However, the optimum catalyst would yield high selectivity with the advantages of heterogeneous systems. Applicants have found that finely divided metal encapsulated in a polysiloxane matrix according to the claimed invention is an effective and selective catalyst for hydrogenation and oxidation reactions at room temperature. The material is both water and air stable.
Silanes have been used in the presence of homogeneous palladium catalysts and acid to reduce alkynes, and the reduction of .pi.-allyl palladium species to olefins can be effected using siloxane reagents. It has also been reported that platinum(II) complexes are reduced with triethoxysilane to form platinum(0) colloids and molecular hydrogen. These platinum(0) colloids are active hydrosilylation catalysts. However, the hydrogenated product obtained by these methods contains metal so finely dispersed that it cannot be removed, even by gel filtration. Instead, chromatography or distillation must be used to remove the metal from the final product.